Controls for hydraulic pumps to vary the maximum flow with pressure are generally complex and because of physical space limitations are available only on larger pumps. In portable airless painting equipment the pumps are generally small and thus it is common practice to use a simple pressure compensated pump. It is then necessary to build different machines for high volume/low pressure and for low volume/high pressure applications. A hydraulic pump which would automatically and continuously vary the maximum flow in inverse proportion to the maximum pressure would allow the use of one machine to cover the full range of outputs from low flow/high pressure to high flow/low pressure without overloading the power source and at the same time to utilize a high percentage of the maximum available power over the full range. The subject invention covers a simple and inexpensive method of accomplishing this type of control in a pressure compensated hydraulic pump.
The pressure compensated hydraulic piston pump is widely used in industry as a means of driving a wide range of hydraulic devices. The pressure compensated pump delivers a fixed maximum volume of fluid at pressures below the design level and then an abrupt cutoff of the flow as the design pressure level is reached. Such pumps are usually equipped with a variable pressure control which allows the cutoff pressure to be easily adjusted. They can also be equipped with a volumetric control, usually in the form of a hand wheel, a lever or an adjusting screw. The volumetric control allows the maximum displacement of the pump to be varied independently of the pressure control.
One such type of pump is shown in FIG. 1 This pump is of the axial piston type and is representative of the prior art.
The operation of the pump can be described as follows. The pump is driven by an external power source through shaft (1. A cylinder block 2 is connected to and rotates with the shaft. A series of pistons 3 in the cylinder block rotate with the block and rest at the right hand side against a tilted swash plate 4 which is located in the pump housing 5.
As the cylinder block rotates the pistons move back and forth in it with their displacement controlled by the angle of the swash disc 4 to the axis of the drive shaft 1. Valve orifices are located in a plate 6 to allow entry of fluid into the pump on the backstroke of the pistons and out of the pump on the forward stroke of the pistons. The inlet and outlet to the pump (not shown) are located in the end cap 7. At a fixed rotational speed and fixed swash plate angle the pump will deliver a constant flow of fluid at varying pressures. A pressure compensating valve generally designated 8 senses the hydraulic system pressure and as the pressure approaches the maximum operating pressure selected, opens conduit 9 to a flow of fluid under pressure. Conduit 9 is connected to cavities 10 and 11 through an opening generally designated 12 in a piston 13. The piston 13 is fixed in position in the end cap 7. A movable cylinder 14 slides over the piston and is in contact with the end of the swash plate 4. The introduction of fluid under pressure into conduits 9, 10, 11 and 12 by the action of the pressure compensating valve will exert a force on the cylinder 14 and when the pressure becomes high enough to overcome the force of spring 15 will move the swash plate to the neutral position and pumping will cease.
An adjustment screw 16 is connected through a rod 17 to contact the inner end of cylinder 14. Movement of the adjusting screw inwards will cause the rod 17 to limit the travel of cylinder 14 to the left and thereby limit the maximum swash plate angle and hence flow. The screw 16 therefore provides an independent control of the maximum output of the pump. The adjusting screw function is commonly incorporated into a hand wheel or lever control.
The theoretical operating characteristics of a pump of this type are generally described in FIG. 2. As the pressure increases to Pmax the maximum volume remains constant at Vmax and then abruptly drops to zero. The actual operating characteristics will vary slightly from theoretical with leakage and the sharpness of the cutoff of the compensator valve.
The horsepower required to deliver a volume of fluid at pressure is directly proportional to the flow times the pressure or EQU HP=KVP
The horsepower required at mix flow will therefore vary linearly from 0 to a maximum value as the pressure increases as shown in FIG. 2.
A hydraulic system driving such a pump can only utilize the maximum horsepower of the power source at the maximum pressure and flow condition and will utilize considerable less than the maximum horsepower at lower pressures. In many applications it would be highly desirable if the maximum output flow could be increased as the pressure drops below Pmax so as to utilize a greater portion of the available horsepower over a portion of the operating range.
In FIG. 3 the dotted curve indicates the way in which the flow would vary for pressures below the maximum if the full horsepower of the source were utilized.